doc: Correct description of values used in FSM for indexes
The implementation of FSM for indexes is simpler than heap, where 0 is
used to track if a page is in-use and (BLCKSZ - 1) if a page is free.
One comment in indexfsm.c and one description in the documentation of
pg_freespacemap were incorrect about that.
Author: Alex Friedman <alexf01@gmail.com>
Discussion: https://postgr.es/m/
71eef655-c192-453f-ac45-
2772fec2cb04@gmail.com
Backpatch-through: 13